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.Dd July 3, 1990
.Dt FD 4FS
.Os
.Sh NAME
.Nm fd ,
.Nm stdin ,
.Nm stdout ,
.Nm stderr
.Nd file descriptor files
.Sh DESCRIPTION
These files, conventionally called
.Pa /dev/fd/0 ,
.Pa /dev/fd/1 ,
.Pa /dev/fd/2 ,
and so on, refer to files accessible through file descriptors.
If file descriptor
.Em n
is open, these two system calls have the same
effect:
.Bd -literal -offset indent
fd = open("/dev/fd/<n>", mode);
fd = dup(n);
.Ed
.Pp
On these files
.Xr creat 2
is equivalent to
.Xr open 2 ,
and
.Fa mode
is
ignored.
As with
.Xr dup 2 ,
subsequent reads or writes on
.Em fd
fail unless the original file descriptor allows the operations.
.Pp
For convenience in referring to standard input, standard output, and standard
error, an additional set of names is provided:
.Pa /dev/stdin
is a synonym
for
.Pa /dev/fd/0 ,
.Pa /dev/stdout
for
.Pa /dev/fd/1 ,
and
.Pa /dev/stderr
for
.Pa /dev/fd/2 .
.Sh DIAGNOSTICS
.Xr open 2
returns
.Sy -1
and
.Va errno
is set to
.Er EBADF
if the associated file descriptor is not open.
.Sh SEE ALSO
.Xr creat 2 ,
.Xr dup 2 ,
.Xr open 2
